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Cat 5e cables and networking



 
 
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  #11  
Old April 8th 04, 05:12 AM
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Opticreep wrote:

But that still doesn't make sense. I don't think the signal-to-noise
ratio should be affected so drastically from this one little mistake.


What you think has no bearing on physics. Mispairing the signals
eliminates essentially *all* of the noise immunity that's designed in to
Cat5 cables. This is undoubtedly the source of your problem and why we
always recommend buying commercially manufactured patch cables rather
than making them yourself (or having a friend make them).

And besides, why would this 24-ft CAT5e cable work between a DSL
straight to my PC, but *not* work between a router and a PC? Maybe it
has to do with signal strength or the different impedences, but
thinking too much makes my head hurt.


More likely it's because your DSL modem only supports 10BaseT, so your
PC to DSL modem connection is only running at 10Mb whereas the router
and your PC probably both support 100BaseTX and thus are trying
(unsuccessfully) to run at 100Mb when they're connected together.

-Larry Jones

I hate it when they look at me that way. -- Calvin
  #12  
Old April 8th 04, 11:39 AM
Opticreep
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Rick Wintjen wrote in message om...
The router probably has a 10/100 interface, also your NIC, so they would
have tried to operate at 100T. The modem is almost certainly a 10T.
Mis-matched pairs are tolerated better at the lower speeds, but you may
have had data errors that weren't bad enough to get your attention.



I switched around the mismatched pairs, and now the problem is gone

Thanks to everyone for their help.
  #13  
Old April 8th 04, 02:29 PM
Emil Luca
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The speed setting at your computer and router should match are at the best
be set to Auto.
Mismatch speeds usually don't work well at all.


wrote in message
...
Opticreep wrote:

But that still doesn't make sense. I don't think the signal-to-noise
ratio should be affected so drastically from this one little mistake.


What you think has no bearing on physics. Mispairing the signals
eliminates essentially *all* of the noise immunity that's designed in to
Cat5 cables. This is undoubtedly the source of your problem and why we
always recommend buying commercially manufactured patch cables rather
than making them yourself (or having a friend make them).

And besides, why would this 24-ft CAT5e cable work between a DSL
straight to my PC, but *not* work between a router and a PC? Maybe it
has to do with signal strength or the different impedences, but
thinking too much makes my head hurt.


More likely it's because your DSL modem only supports 10BaseT, so your
PC to DSL modem connection is only running at 10Mb whereas the router
and your PC probably both support 100BaseTX and thus are trying
(unsuccessfully) to run at 100Mb when they're connected together.

-Larry Jones

I hate it when they look at me that way. -- Calvin



  #14  
Old April 12th 04, 07:12 AM
glen herrmannsfeldt
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Opticreep wrote:
(snip)

It's straight-through.


Although I may have a weird theory on why I'm having problems.
Apparently, my CAT5 cables weren't properly made. I noticed that pins
3 & 6 on the RJ45 didn't make a twisted pair. Instead, the techie who
installed the RJ45's for me made twisted pairs out of pins 3 & 4, and
then 5 & 6. This probably generated a lot of noise on signals going
through pins 3 and 6. I'm quite sure that category 5 standards state
that pins 3 & 6 make up a pair (and so does 4 & 5).


It isn't so much signal to noise, but the signal couples to the
other wires. Depending on which wires, it can make a big
difference in the signal at the other end.

This oversight probably didn't affect my short 0.5 meter cables too
much. But on a 25-meter cable, the signal-to-noise ratio might have
become too high. At least that's my theory.


But that still doesn't make sense. I don't think the signal-to-noise
ratio should be affected so drastically from this one little mistake.
And besides, why would this 24-ft CAT5e cable work between a DSL
straight to my PC, but *not* work between a router and a PC? Maybe it
has to do with signal strength or the different impedences, but
thinking too much makes my head hurt.


All routers I know of have 10baseT input and 10/100 output. That
makes a big difference in how well the cable works if mispaired.

-- glen

  #15  
Old April 12th 04, 11:43 AM
James Knott
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glen herrmannsfeldt wrote:

It isn't so much signal to noise, but the signal couples to the
other wires. Depending on which wires, it can make a big
difference in the signal at the other end.


Anything other than the desired signal is noise. Therefor signal from the
other wires is noise. There's even a word for that type of noise. It's
called crosstalk.

--

Fundamentalism is fundamentally wrong.

To reply to this message, replace everything to the left of "@" with
james.knott.
  #16  
Old April 12th 04, 07:42 PM
glen herrmannsfeldt
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James Knott wrote:

glen herrmannsfeldt wrote:


It isn't so much signal to noise, but the signal couples to the
other wires. Depending on which wires, it can make a big
difference in the signal at the other end.


Anything other than the desired signal is noise. Therefor signal from the
other wires is noise. There's even a word for that type of noise. It's
called crosstalk.


Well, it is, but neither describes this case so well.

If you pair on (3,4) and (5,6) for 100baseTX the (3,6) signal
will couple to wires 4 and 5. Most likely the result is that
there isn't enough signal left at the end, even without any
noise. The receiver requires a certain amount of signal,
independent of the amount of noise actually present.

Depending on the arrangement of the pairs in the cable,
some will couple as common mode signals onto other pairs,
again lost to the intended receiver.

For a different example, if you manage to pair (1,3) and (2,6)
the signal will then couple between the active signaling
pairs. Though even in this case it should be absorbed by
the terminating resistors on the transmitter and not affect
the receiver much (for 100baseTX).

Now for gigabit, where all four pairs are used in both directions,
any coupling goes directly into the receiver of the wrong pair.
I don't believe that the OP is using gigabit, though.

-- glen

 




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